Refrigerant vapor compression systems are well known in the art and commonly used for conditioning air (or other secondary media) to be supplied to a climate-controlled comfort zone within a residence, office building, hospital, school, restaurant or other facility. Refrigerant vapor compression systems are also commonly used in transport refrigeration for cooling air supplied to a temperature-controlled cargo space of a truck, trailer, container or the like for transporting perishable or frozen items, and in commercial refrigeration for cooling air supplied to a temperature-controlled space in a cold room, a beverage cooler, a diary case or a refrigerated merchandiser for displaying perishable food items in a chilled or frozen state, as appropriate. Typically, these refrigerant vapor compression systems include: a compressor, a heat rejection heat exchanger, an evaporator; and an expansion device. Commonly, the expansion device, typically a fixed orifice, a capillary tube, a thermostatic expansion valve (TXV) or an electronic expansion valve (EXV), is disposed in the refrigerant line upstream, with respect to refrigerant flow, of the evaporator and downstream of the condenser. These basic refrigerant vapor compression system components are serially interconnected by refrigerant lines in a closed-loop refrigerant circuit, arranged in accord with known refrigerant vapor compression cycles. The heat rejection heat exchanger functions as a refrigerant vapor condenser in subcritical cycles and a refrigerant vapor cooler in transcritical cycles.
To improve performance of the refrigerant vapor compression system and to control the temperature of the refrigerant vapor discharged from the final stage of the compressor over a wide range of operating conditions, it is known to equip such systems with an economizer cycle incorporating a refrigerant-to-refrigerant economizer heat exchanger. The economizer heat exchanger is generally disposed in the refrigerant circuit intermediate the condenser and the evaporator, with respect to refrigerant flow. In the economized mode of operation, at least a portion of the refrigerant leaving the condenser is diverted from the primary refrigerant circuit, expanded to an intermediate pressure and then passed through the economizer heat exchanger in heat exchange relationship with the main portion of the refrigerant leaving the condenser. In this manner, any liquid in the economized expanded refrigerant flow is typically evaporated, and then the economized refrigerant flow is typically superheated, while the refrigerant passing through the primary refrigerant circuit from the condenser to the evaporator is further cooled. Typically, the expanded refrigerant vapor is injected into an intermediate stage in the compression process, either through an injection port or ports opening into an intermediate pressure stage of the compression chamber (or chambers) of a single compressor or, in the case of a multiple compressor system, into a refrigerant line extending between the discharge outlet of the upstream compressor and the suction inlet of the downstream compressor.
Conventional refrigerant vapor compression systems, whether economized or non-economized, often include a suction modulation valve (SMV) that is interdisposed in the refrigerant circuit downstream, with respect to refrigerant flow, of the evaporator and upstream, with respect to refrigerant flow, of the suction inlet to the compressor. The suction modulation valve functions under the direction of the system controller to throttle refrigerant flow through the compressor and subsequently through the evaporator, by reducing the refrigerant pressure at the suction inlet to the compressor (suction inlet pressure). In operation, when a reduction in system capacity is desired, the system controller selectively further closes the SMV to reduce refrigerant flow to the compressor. Conversely, when an increase in system capacity is desired, the system controller selectively further opens the SMV to increase refrigerant flow to the compressor.
Although the SMV may be positioned fully opened when the system is operating at or near its maximum capacity, in conventional refrigerant vapor compression systems, the SMV cannot be positioned fully closed or even nearly fully closed due to resultant problems. For example, a minimum suction inlet pressure is required for proper operation of the compressor. If the suction inlet pressure was to fall below this minimum threshold pressure, such as would result from closing the SMV down too much, the compressor would overheat and oil delivery by the oil pump with the compressor could be comprised. Additionally, the mass flow rate of the refrigerant circulating through the refrigerant circuit could become so low that oil would be retained within the evaporator or in the suction line upstream of the compressor, rather than entering the compressor, which ultimately could lead to substantially all of the oil being pumped out of the compressor and consequent compressor failure. Therefore, in conventional refrigerant vapor compression systems, desired control of refrigerant flow through the evaporator to very low or even zero flow may not be achievable, thereby limiting the ability to attain tight temperature control in the controlled environment with which the evaporator is associated. In the prior art, the refrigerant vapor compression system would cycle on and off to obtain time-averaged near zero capacity, which is undesirable from the reliability and temperature control perspectives.
U.S. Pat. No. 6,058,729 discloses a method of optimizing cooling capacity, energy efficiency and reliability of an economized refrigerant vapor compression system for a transport refrigeration unit when operating at or near maximum capacity, during the pulldown, of product temperature within the associated storage container. The disclosed refrigerant vapor compression system incorporates a refrigerant-to-refrigerant heat exchanger into the refrigerant circuit as an economizer. The disclosed system also includes a suction modulation valve (SMV) for throttling refrigerant flow to the suction inlet of the compressor and an intermediate pressure-to-suction pressure unloading circuit for compressor capacity control.
U.S. Pat. No. 7,114,349 discloses a refrigerant vapor compression system with a refrigerant-to-refrigerant heat exchanger interdisposed in the refrigerant circuit downstream of the condenser, with respect to refrigerant flow, and upstream of the evaporator, with respect to refrigerant flow. Through various bypass lines and manipulation of various open/closed solenoid valves associated with the bypass lines, this heat exchanger may be operated either as an economizer heat exchanger or as a liquid-suction heat exchanger. When the system is operating with the refrigerant-to-refrigerant heat exchanger functioning as an economizer, refrigerant is passed from the primary refrigerant circuit through an economizer expansion device and thence through the refrigerant-to-refrigerant heat exchanger in heat exchange relationship with the main portion of the refrigerant passing through the primary refrigerant circuit from the condenser to the evaporator. After traversing the refrigerant-to-refrigerant heat exchanger, the expanded refrigerant is injected into an intermediate pressure stage of the compressor or returned to the primary refrigerant circuit at a point downstream, with respect to refrigerant flow, of the evaporator and upstream of the suction inlet of the compressor. In the disclosed system, the conventional suction modulation valve is replaced by an open/closed solenoid valve, which may be selectively closed to prevent refrigerant flow from passing directly from the evaporator outlet to the suction inlet of the compressor, and divert that flow to pass through the refrigerant-to-refrigerant heat exchanger prior to passing into the compressor suction inlet.